It's a word fuck up. It's occurs when you are pist or taken by shock, and you mean to either say "fuck it all" or "what the fuck" but in your state of shock you actually say "fuck the wall" a bit of a combination. And then it becomes the phrase that your friends constantly remind you that you accidently muttered in your moment of shock.

Claudia: hey Liv, did you know it was Moms birthday yesterday...
Liv: "fuck the wall! are you for real?